ABSTRACT ? CORE A Core A will perform testing to evaluate post-transplant immune reconstitution in patients (supporting Project 1) and in X-SCID and PK dogs (supporting Projects 2 & 3) undergoing hematopoietic cell transplantation (HCT) or gene therapy. The Core will evaluate effector T cell, regulatory T cell, and B cell recovery and differentiation, will measure serum BAFF & April levels, and will evaluate immune cell signaling and function. In addition, the Core will work with the Seattle Children's Immunology Diagnostic Lab (IDL) to evaluate functional immunity in patients and transplanted dogs by measuring disease-specific recovery of immune function and responses to vaccination with the neoantigen Bacteriophage ?X174. The core is designed to fully support the proposed observational studies of immune reconstitution after HCT in up to 30 patients each year for project 1 and in all transplanted and gene-therapy treated X-SCID and PK dogs for projects 2 & 3. Data from these analyses will be used to define the course and extent of post-HCT immune reconstitution and will be correlated with the results from clinical evaluations, laboratory testing, graft outcomes, and GVHD to determine the efficacy of the proposed transplant and gene therapy approaches outlined in projects 1, 2, and 3.